Conventionally, as one type of image processing for making an input image appear more beautiful, brightness conversion processing is commonly used. In such brightness conversion processing, brightness conversion is performed by calculating, for example, based on a brightness histogram for an entire image, a brightness conversion coefficient common to the entire image region and multiplying each pixel by the brightness conversion coefficient.
Specifically, the brightness histogram (frequencies in brightness ranges, that is, the distribution of the number of pixels falling within each predetermined brightness range) for the entire input image is first generated. Then, the brightness conversion coefficient is calculated based on this brightness histogram such that brightness ranges with higher frequencies in the input image are made wider in an output image, and in contrast, brightness ranges with lower frequencies in the input image are made narrower in the output image. Thus, the brightness conversion coefficient is different in each brightness level or each predetermined brightness range.
Thus, although portions of the input image in a brightness range of lower frequency have a lower contrast (brightness difference), portions of the input image in a brightness range of higher frequency have a higher contrast. Hence, it is possible to achieve a higher contrast than that of the input image in terms of an entire image.
Conventional technologies related to the foregoing are disclosed in patent documents 1 to 4 and other documents.
Image processing devices are disclosed and proposed in patent documents 5 and 6 that perform predetermined reduction processing on data of an input image and generate a brightness histogram.
An image signal movement detection circuit is disclosed and proposed in patent document 7. The image signal movement detection circuit has threshold processing means that compares a frame difference signal of an input image signal with a threshold to output the result of the comparison as a movement detection signal and means for detecting the average brightness of an entire screen on image signals (or means that divides image signals into a plurality of blocks to detect the average brightness on a block-by-block basis), and is configured such that the threshold of the threshold processing means is appropriately switched based on the output of the means for detecting the average brightness.
Patent document 1: JP-T-2004-530368
Patent document 2: JP-A-2000-013625
Patent document 3: JP-A-2000-036043
Patent document 4: JP-A-2004-145399
Patent document 5: JP-A-H10-198802
Patent document 6: JP-A-2001-268400
Patent document 7: JP-A-H08-046925